This application is a 371 of PCT/FR97/00524 May 25, 1997.
The present invention concerns a radio station, which may be used particularly as a base station in cellular radio systems.
Traditionally, radio-communications systems with mobile radio stations use base stations equipped with vertically and linearly polarised antennas. When it is required to couple more than one radio signal source to an antenna, devices of the hybrid coupler type with only one output connected to the antenna are used. In this case, the other hybrid coupler output must be connected to a load resistor to match the impedance. This load resistor dissipates half the radio power which is not radiated usefully and causes an undesirable rise in temperature.
One drawback of the use of linear polarisation in radio-communications with mobile radio stations is that the quality of communication depends on the orientation of the mobile radio station. For example, measurements have shown that a vehicle antenna of the coaxial type inclined at 45xc2x0 may cause 80% signal loss for a vertical linear polarisation transmission.
Moreover, it is known that diversity treatments enable the performance of radio-communication systems to be improved. Base stations of cellular systems normally use spatial type diversity in reception, by means of two spatially separate vertically polarised antennas. The use of polarisation diversity rather than spatial diversity has also been suggested. For this two antennas located in the same place are used, one sensitive to vertical polarisation and the other sensitive to horizontal polarisation.
One object of the present invention is to improve the possibilities offered for transmission by a radio communication base station.
The invention thus proposes a radio station, including two antennas combined respectively with first and second hybrid transmission polarisation couplers, each antenna being arranged to generate two orthogonal electric field components in response to two respective quadrature radio signals from the corresponding polarisation coupler. The station further includes at least one hybrid distribution coupler with a first output connected to a first input of the first polarisation coupler and a second output connected to a first input of the second polarisation coupler, and at least one radio signal source delivering a radio signal to a first input of the distribution coupler.
Thus, each antenna transmits a portion of the radio signal from the circularly polarised source. As a result, the quality of the reception by the mobile radio station no longer depends on the orientation of its antenna in relation to a linear polarisation direction.
In a first version of the invention, the hybrid couplers are connected to each other and to the antennas in such a way that the radio signal from the source is transmitted by the two antennas in the form of two respective radio waves circularly polarised in the same direction. An appropriate relative positioning of the two antennas, and an appropriate choice of length of the coaxial cables connecting the couplers to each other then enables a gain in transmission directivity (up to about 3 dB) to be obtained. Such a gain in directivity means that the base station is highly suitable for the microcellular case, especially when radio penetration inside buildings is required.
In another version of the invention, the hybrid couplers are connected to each other and to the antennas in such a way that the radio signal from the source is transmitted by the two antennas in the form of two respective radio waves circularly polarized in opposite directions. This gives a transmission polarisation diversity which enables the effects of fading to be counteracted. This version is very appropriate in cases where the propagation medium creates relatively little diversity, i.e. when the waves transmitted sustain relatively few reflections (propagation in a rural, desert, maritime environment etc). The gain in diversity thus obtained can run from 3 to 10 dB.
It is noted that it is very easy to switch from one of the versions mentioned above to the other, simply by modifying the connections of the coaxial cables connected to the couplers.
The same type of equipment can thus be used to meet the diverse requirements of the network operator.
The advantages disclosed above can easily be obtained for several radio signal sources. The two coupling stages have the advantage, when several radio sources are connected, of enabling all the available power (except for losses in duplexers) to be radiated usefully, which avoids useless heat being dissipated in the rack.
The use of the two antennas of the invention further enables advantageous arrangements in the reception part of the radio station. These arrangements are combined to advantage with those which have just been mentioned for the transmission part, but they would be applicable independently. In accordance with one of these arrangements, the radio station includes at least one receiver ensuring a diversity treatment of two input radio signals, one of the said input radio signals being obtained from an electric field component picked up by one of the two antennas according to a first direction, and the other input radio signal being obtained from an electric field component picked up by the other antenna according to a second direction orthogonal to the said first direction.
The receiver then combines the advantages of both spatial diversity and polarisation diversity in counteracting fading. It is possible to install several receivers in this way.